1. Field of the Invention
This invention relates to analog-to-digital converters and, more particularly, to a sigma-delta type analog-to-digital converter constructed from superconducting devices operating at gigahertz frequencies.
2. Background Information
Conventional analog-to-digital (A/D) converters repetitively sample and hold the analog signal for application to a series of comparators and or analog processors. In such converters, at least one and in some cases as many as 2.sup.n (where n is the number of bits) precision analog circuits are required to produce the digital output signal. Furthermore, these conventional A/D converters produce a significant quantization error in the least significant bit and may sacrifice other parameters such as speed, power, weight, or life characteristics.
An improved type of A/D converter known as the sigma-delta A/D converter utilizes an integrator to which the analog signal is applied, a single rough quantizer operating at high speed to convert the output of the integrator to a single bit digital signal, a digital filter which converts the high speed single bit output of the quantizer into a multi-bit digital output, and a feedback loop including the quantizer, a digital-to-analog converter and the integrator. The quantizer samples the integrator output at a rate many times the Nyquist rate. The resolution of the digital signal is a function of the oversampling rate of the quantizer.
Typically, sigma-delta converters have been implemented in integrated circuits with sampling rates of about 3 MHz, although there have been reports of devices operating at 60-80 MHz. Sampling at 3 MHz on a 500 Hz signal produces a 17 bit digital output signal at the Nyquist rate. With 80 MHz sampling 24 bit resolution of the 500 Hz signal could be achieved. The feedback in the sigma-delta converter integrates the error in the least significant digit thereby shifting the noise to higher frequencies above the fundamental frequency of the analog input signal. The chief advantage of the presently available sigma-delta converters is that they substitute high speed digital signal processing for high-precision analog circuits.
A converter utilizing a superconducting quantum interference device (SQUID) to generate pulses at frequencies of up to 30 GHz for an analog-to-digital conversion has been suggested. The SQUID incorporates Josephson junctions which generate a single flux quantum (SFQ) pulse train that is used to clock a series of superconducting flip-flops. A separate superconducting flip-flop is required for each digit of the digital output signal. As in the case of the conventional analog-to-digital circuit which it resembles, this converter has significant quantization error in the least significant bit.
High-dynamic range analog-to-digital converters are important in surveillance radar, signal interception, medical imaging and focal plane array applications.
There is a need for improved analog-to-digital for these applications and others.
More particularly, there is a need for analog-to-digital converters with improved dynamic range.
There is also a need for analog-to-digital converters with improved accuracy.
There is a further need for improved analog-to-digital converters which do not require extensive precision analog circuitry.